1. Field of the Invention
The present invention relates to a fuel cell, and more particularly, to a fuel cell having a structure enabling detection of a wet state (condition) of the fuel cell.
2. Description of the Related Art
A fuel cell apparatus has a potential of realizing a suppliable energy amount per unit volume, which is several to ten times higher than that of a related art battery.
Further, by charging a fuel, a long-term continuous use is enabled, so there is expected a wide range of application, for example, to a small electronic devices such as mobile phones and notebook personal computers.
Among the fuel cell apparatuses, a polymer electrolyte fuel cell is advantageous in that it can be used at temperature close to room temperature, and in the polymer electrolyte fuel cell, an electrolyte is in a solid state instead of a liquid state, so the polymer electrolyte fuel cell can be safely carried.
In the polymer electrolyte fuel cell, a fuel cell unit has a structure in which a polymer electrolyte membrane is sandwiched between a fuel electrode and an oxidizer electrode each having a catalyst layer.
To the fuel electrode, for example, hydrogen is supplied as a fuel. To the oxidizer electrode, for example, oxygen in air is supplied as an oxidizer.
The polymer electrolyte membrane in the polymer electrolyte fuel cell contains water therein to conduct hydrogen ions.
However, ion conductivity which affects performance of the polymer electrolyte fuel cell depends on a degree of humidity of the polymer electrolyte membrane.
That is, when the polymer electrolyte membrane is too dry, the ion conductivity is remarkably reduced, thereby reducing an output of the fuel cell due to increase in internal resistance. This phenomenon is called dryout.
Accordingly, in order for the polymer electrolyte fuel cell to perform power generation, it is necessary that the polymer electrolyte membrane for conducting ions be wet to an appropriate degree.
On the other hand, the hydrogen ions which have passed through the polymer electrolyte membrane and the oxidizer (oxygen) react with each other in the oxidizer electrode, and water is generated in the oxidizer electrode.
Accordingly, moisture (water vapor) is contained in an exhaust gas of the fuel cell. When the moisture in a liquid state becomes excessive due to the generated moisture, a flow of the fuel or the oxidizer required for the fuel cell reaction is inhibited, so a stable output cannot be obtained. This phenomenon is called flooding.
Regarding those phenomena, for an operation of the fuel cell, it is necessary that the wet state be maintained appropriately. Accordingly, conventionally, Japanese Patent Application Laid-Open No. 2002-164065 suggests a fuel cell as described below as a technology of appropriately controlling the wet state of the fuel cell.
In the fuel cell, in an alignment direction of cell units constituting a fuel cell stack, relative humidity distribution is formed and a comparison is made between a voltage of the cell unit having low relative humidity and a voltage of the cell unit having high relative humidity, thereby determining the wet state of the fuel cell.
However, in the fuel cell as disclosed in Japanese Patent Application Laid-Open No. 2002-164065, in order to determine the wet state of the fuel cell, it is necessary that the humidity distribution be formed in the alignment direction of the cell units of the fuel cell stack.
As described above, with the structure in which the relative humidity distribution is formed in the alignment direction of the cell units, when determination is made such that the wet state of the fuel cell is not appropriate, reduction in power generation efficiency of the fuel cell has been already caused.
Therefore, the determination that the wet state of the fuel cell is not appropriate is made after the power generation efficiency of the fuel cell is reduced or while the power generation efficiency is being reduced. Accordingly, it is difficult to make the determination before occurrence of the reduction in power generation efficiency as described above.
For example, in a case where the relative humidity distribution is formed in the fuel cell stack with a gas inflow direction and a coolant water inflow direction being made opposite to each other, when it is determined that a sufficient wet state is not achieved, the power generation efficiency of the fuel cell unit positioned on a gas inlet side has already been reduced.
Further, in the fuel cell as disclosed in Japanese Patent Application Laid-Open No. 2002-164065, even in the fuel cell unit in which the power generation efficiency has not been reduced yet at a time when the determination is made such that the wet state of the fuel cell is not appropriate, conditions are extremely close to conditions in which the reduction in power generation efficiency is caused. Accordingly, in the related art fuel cell, in a period of time from a time at which the reduction in power generation efficiency is detected to a time at which some countermeasure is taken, there is a high risk of the power generation efficiency of more fuel cell units being reduced and the power generation efficiency of the fuel cell as a whole be reduced to a larger degree.